Hi Nigel,
Yes, we always advise updating to the latest software version, as this contains fixes and/or improvements to expand or increase the efficiency of the system. I particular, the last update (20180129) included several improvements to make Mode C Detection more effective, as well as streamlining of the PilotAware audio alerts and a few other 'fixes'.
What software version are you currently running? You need to have at least version 20160820 or later installed for USB updating to work as this was the first version that contained the necessary USB Stick Reader software. Other potential issues could include the USB stick not being the correct format - it must be formatted as FAT32, the update file must be in the 'root' (top level) of the USB stick - i.e. NOT inside a folder and it MUST be named PilotAware.pgp (though the .pgp file extension is not always visible, depending on your computer settings.
Hope this helps, if not get back to me.
Regards
Peter